This invention relates generally to the measurement of curves, and more particularly to a guage suited for measuring the curvature of lapping tools used in the manufacture of lenses for glasses or the like.
In the manufacture of lenses for eyeglasses or the like, it is customary to produce the final curvature of the lens with a lapping tool provided with the necessary curvature to produce the shape required by the prescription for the lens. The accuracy of the lens is ultimately determined by the accuracy of the curvature provided on the lapping tool. It is therefore necessary to provide means for accurately determining such tool curvature. Further, such lapping tools are often provided with two curvatures with one uniform curvature or base curvature extending in a first direction and a second uniform but different curvature or cross curvature extending in a second direction perpendicular to the first direction. In the past, the curvature has been measured by accurately formed metal gauge templates, which are formed with an edge having a known curvature. Such templates are held against the lapping tool, with the template having the first desired curvature held against the tool in alignment with the first direction to determine the first curvature and a second template having the second desired curvature held against the tool in alignment with the second direction to measure the second curvature.
The measurement with such templates leads to errors, since the fit of the template against the surface of the tool must be visually determined. Further, any misalignment between the plane of the template and the first and second directions introduces erroneous measurements.
More recently, gauges have been employed wherein an accurate dial indicator is mounted on a base, with its movable dial indicator post located midway between and in a plane containing a pair of fixed posts. In such gauges, means are provided to support the lapping tool on the gauge and to move the three posts into engagement with the surface of the tool. The curvature is determined with such a gauge by determining the relative positions of three points along the curve which are a known distance apart. With the information concerning such three points along the curve provided by the gauge, the radius of curvature is mathematically determined.
Such support for the tool is usually arranged to accept the tool in two positions. In the first position, the tool is supposed to be positioned with the first direction or axis aligned with the plane of the posts and in the other position, the second direction or axis is supposed to be aligned with the plane of the posts. Such gauges, if accurately manufactured and calibrated, provided, in theory, accurate measurement of the two curvatures. However, in practice erroneous readings occur because the axes of the curves on the tool are not necessarily positioned in the plane of the three posts and the gauge does not consistently measure the curvature at an equator but, in practice, often measures the curvature along a plane spaced from the equator.